Crystal structure studies of tetragonal sodium tungsten bronzes,NaxWO3. I. Na0.33WO3 and Na0.48WO3

The crystal structures of two sodium tungsten bronzes, Na_0.33WO₃ and Na_0.48WO₃, have been determined by three-dimensional single-crystal X-ray analysis. They were found to crystallize in the tetragonal space groups $P\overline{4}{2}_{1}m(a=12.097,c=3.754\AA ,Z=10)$ and P4/mbm (a = 12.150,c = 3.769Å, Z = 10), respectively. The structures were solved by standard Patterson and Fourier techniques and refined by full-matrix least-squares to final conventional discrepancy indices of 8.9% for Na_0.33WO₃ and 8.4% for Na_0.48WO₃. In general, the oxygen atoms were found to be either twofold or fourfold disordered, suggesting that the WO₆ octahedra do not have axes exactly aligned parallel to the crystallographicc-axis. The structure found can be viewed as a composite of two kinds of domain structures. These domain structures would require a doubling of thec-axis along with selection of newa- andb-axes along the [1 1 0] and [$[\overline{1}10]$] directions. There exist pentagonal and tetragonal sites in both these sodium tungsten bronzes for sodium atoms occupancy. In Na _xWO₃, x = 0.48, all the pentagonal sites are filled and 40% of the smaller tetragonal sites are also occupied. As x decreases to 0.33 though, only the pentagonal sites are occupied. A relation between the x value and the Na _xWO₃ crystal structures is postulated, extrapolating from the results found in these structure determinations.